In response to the demand for more reliable and higher capacity data storage and retrieval systems, there is considerable activity in the research and development of so-called optical disk recording systems. These systems utilize a highly focused modulated beam of light, such as a laser beam, which is directed onto a recording layer which is capable of absorbing a substantial amount of the light. The heat thusly produced causes the light-absorbing material in the areas struck by the highly focused laser beam to change chemically and/or physically, thus producing a concomitant change in optical properties, e.g., transmissivity or reflectivity, in the affected area. For readout, the contrast between the amount of light transmitted or reflected from the unaffected parts of the absorbing layer and from the marked areas of the layer is measured. Examples of such recording systems are disclosed throughout the literature and in numerous U.S. Patents such as U.S. Pat. Nos. 3,314,073 and 3,474,457. In recording data, a rotating disk having a light-absorptive recording layer is exposed to modulated radiation from a laser source. This radiation is passed through a modulator and appropriate optics, and the highly focused laser beam is directed onto the disk which forms by chemical and/or physical reaction of the light-absorbing layer a series of very small marks along a circular path within the light-absorptive layer. The frequency of the marks is determined by the modulator inputs. Using laser beams with a focused spot diameter of 1 .mu.m or less, data can be stored at a density of 10.sup.8 bits/cm.sup.2 or higher.
The simplest optical disk medium consists merely of a dimensionally stable solid substrate on which is coated a thin layer of light-absorptive material such as a metal layer. When the light-absorptive layer is struck by an intense beam of coherent light, such as from a laser source, the light-absorptive material is either vaporized and/or thermally degraded, thereby producing a very small marked area which exhibits different transmissivity or reflectivity than the adjacent unmarked layer. Multilayer antireflection structures, such as those disclosed in U.S. Pat. No. 4,305,081 to Spong and U.S. Pat. No. 4,270,132 to Bell, increase the absorption of the laser beam which also gives better read/write contrast than with the use of simple single layer media. Therefore, for purposes of obtaining better power efficiency, sensitivity and readout response of the record, it has been preferred to use multilayer antireflective structures.
There are two basic types of multilayer antireflective structures, one of which is basically a bilayer structure and the other a trilayer structure. In bilayer media, the substrate is coated with a very smooth, highly reflective material such as aluminum, on top of which is coated a layer of moderately light-absorptive material which is preferably of a thickness corresponding to about .lambda./4n, where .lambda. is the wavelength of the recording light source and n is the refractive index of the light-absorptive layer. In trilayer media, the substrate is likewise coated with a first layer of very smooth highly reflective material on which is coated a second layer of transparent material. Atop the transparent second layer is coated a thin third layer of strongly light-absorptive material. The combined thickness of the transparent and absorptive layers is preferably adjusted to be about .lambda./4n. In both types of structures, the adjustment of certain layer thicknesses according to the wavelength of light and refractive index of the layer is for the purpose of minimizing the amount of light reflected from the unmarked areas and maximizing the amount of light reflected from the marked areas, thus producing a higher playback signal amplitude. A detailed discussion of the three types of disk construction is given by A. E. Bell in Computer Design. Jan. 1983, pp. 133-146 and the references cited therein. See especially Bell and Spong, IEEE Journal of Quantum Electronics, Vol. QE-14, 1978, pp. 487-495.
It will be realized, of course, that the terms "bilayer" and "trilayer" refer only to the fundamental optical layers and do not exclude the use of ancillary layers. In particular, it is essential in most instances to have a polymeric layer which serves two important functions: (1) the layer must be optically smooth in order to provide an optically suitable foundation for the overlying reflective layer; and (2) the layer must have good adhesion to the underlying substrate as well as the overlying reflective layer. Furthermore, these properties must persist under all the environmental conditions which may exist as the medium is used and stored.